Cellular wireless is an increasingly popular means of personal communication in the modern world. People are using cellular wireless networks for the exchange of voice and data over cellular telephones, Personal Digital Assistants (“PDAs”), cellular modems, and other mobile stations. In principle, a user can seek information over the Internet or call anyone over a Public Switched Telephone Network (“PSTN”) from any place within coverage of the cellular wireless network.
A typical cellular wireless system includes a number of base stations that radiate to define wireless coverage areas, such as cells and cell sectors, in which mobile stations can operate. In turn, each base station is typically coupled with equipment that provides connectivity with one or more transport networks, such as the public switched telephone network (PSTN) and/or the Internet for instance. With this arrangement, a mobile station operating within a coverage area of any base station can engage in air interface communication with the base station and can thereby communicate via the base station with various remote network entities.
In practice, communications over the air interface between a base station and a mobile station are structured in accordance with a particular air interface protocol or “access technology.” Numerous such protocols are well known in the art, and others will be developed in the future. Examples of existing protocols include CDMA (e.g., 1xRTT, 1xEV-DO), iDEN, TDMA, AMPS, GSM, GPRS, UMTS, EDGE, WiMAX (e.g., IEEE 802.16), LTE, microwave, satellite, MMDS, Wi-Fi (e.g., IEEE 802.11), and Bluetooth. Each protocol may define its own procedures for initiation of calls, handoff between coverage areas, and functions related to air interface communication.
Further, each base station in a cellular wireless system has various air interface resources that the base station can allocate for use to serve mobile stations operating in its coverage area(s). For example, in each coverage area, the base station may have a limited amount of transmission power (e.g., a maximum power level of the base station's power amplifier), and the base station may need to allocate that power among concurrent communications with mobile stations. As another example, in each coverage area, the base station may have a limited frequency spectrum, and the base station may need to allocate portions of that spectrum among concurrent communications with mobile stations. And as still another example, in each coverage area, the base station may have a limited supply of codes to use for encoding air interface communications, and the base station may need to divvy those codes among concurrent communications as well.
As a specific example, each coverage area in a spread spectrum system uses orthogonal spreading codes to uniquely define communication channels on the air interface, and in order to preserve distinctions (orthogonality) between the codes, a limited number of such codes exists. Each sector or other coverage area of a CDMA spread spectrum system, for instance, has a limited set of Walsh codes that are used to define various air interface channels. Typically, a small number of those Walsh codes are reserved for use to encode overhead control channels, while the remainder of the Walsh codes are assigned on an as-needed basis to encode bearer traffic channels for voice or data calls.
As each CDMA sector has a limited number of Walsh codes, each sector can support a limited number of concurrent calls. Furthermore, this limitation becomes more complex because most CDMA systems allow for multiple types of air interface encoding, with each type consuming a different amount of spreading resources as well as a different amount of total available base station power.
Under the well known CDMA2000 standard, for instance, at least two different “radio configurations” are defined—“RC3” and “RC4”. RC3 is typically used for voice calls, and RC4 is typically used for data calls, however a base station may generally select either configuration for a given call.
Each of these radio configurations uses different length Walsh codes that provide different amounts of spreading, and each radio configuration tends to consume a different amount of base station power. In particular, each RC3 call uses a 64-bit Walsh code that provides more spreading and consumes less base station power, while each RC4 call uses a 128-bit Walsh code that provides less spreading and consumes more base station power. In terms of orthogonal coding resources available in a given sector, each 128-bit Walsh code uses about one half the resources of a 64-bit Walsh code. As a result, a given CDMA sector can generally support (i) a particular number of RC3 calls, (ii) twice as many RC4 calls, (iii) or some combination of RC3 calls and RC4 calls. (Additional radio configurations may be available as well, which may further increase complexity.)